Magnetic shield



April 16, 1929.

M. H. BENNETT MAGNETIC SHIELD Filed Dec. 1, 1926 anvmtoz Patented pr.- 16, 11929.

UNITED sra'rs PATENT- ()FFICE.

MORRIS H. BENNETT, OF WATERBURY, CONNECTICUT, ASSIGNGR T0 SCOVILL MANU FACT'URING COMPANY, OF WATER/BURY, CONNECTICUT, A CQRPORATION 0F CON- nnorron'r.

MAGNETIC SHIELD.

Application filed December 1, 192a. Serial No. 15.1,ee5.

This invention relates to magnetic shields and more particularly to shields for pro tecting transformersand coils for use in radio transmitting and receiving sets.

In radioreceiving and transmitting sets it is important that tuning coils, transformers and other delicate apparatus be shielded so that extraneous fields, such as might be set up by local interferences, local broadcasting stations or some adjacent piece of apparatus, be prevented from getting into the coil and adversely affecting its operation. I These extraneous fields tend either to increase or to decrease the si nal received, depending upon their character, and since they may be strong enough to prevent timing out of a particularstation, some means should be providedfor their elimination.

In theprior art, coilshave been shielded by enclosing them in some non-ferrous metal casing in such a manner that these extraneous fields produced eddy current flow within the shield. These eddy currents, in turn, set up counter fields which prevent the major part of the extraneous field from getting through the shield into the coil. It was customary to make these shields cylindrical, square and of various other shapes, in some cases being split to prevent the well known short-circuited turn effect. In all these cases, however, eddy current losses were introduced into the shield itself.

It is, therefore, one of the'principal objects ofthis invention to provide an electro magnetic shield so designed that its shape i and a surface be plotted. so that it includes these points, the shield will follow this shape with the result that, since equal potentials are introduced into the shield at all lmlll'tf, no eddy current flow will occur. Therefore, nolosses from the shielded coil can be introduced into a circuit by induction into the shield.

In the drawings, wherein is illustrated one form of the invention,

Figure 1 is a top view, partly in section, of a coil to which a shield, designed in accordance with this invention, has been applied;

, Figure 2 is a plan view of a portion. of the shield shown Fig. 1; and

Figure '3 is a of the shield. In the specific representation shown in the drawings, a core 1, of insulating material having a primary winding 2 is inserted and fit closely in a second core 1 also of insulating material having a secondary Winding 3, the whole being inserted within a shield t which is formed of two parts, 5 and 6, so designed as to include in its surfaces substantially all points of equal potential. Since the induced potential at one end of the coil is opposite to that at the other end of the coil, it is preferable to split the shield 4 into two parts. These parts may be joined by any suitable insulatingmaterial (for example, paper) 7,

perspective view of a portion means (not shown). Then low resistance 'material is used as a shield it is also preferable to further split the shield longitudinally of the core and secure the parts together by means of insulating material 8. By thus splitting the shield the effect of a short-circuited turn is prevented.

Experience has shown, however, that this longitudinal splitting is usually unnecessary in shields of high resistance material.

It is understood, of course, that, in shaping the shield to conform to the surface of equal potential radiated from the coil, no losses due to induction into the shield have been introduced, this being due to the fact that the coil flux shape is the same as the shield. However, in the case of extraneous fields such as might be set up by local interference, local broadcasting stations or some adjacent piece of apparatus, the field set up by these extraneous interferences is always of some shape different from that of the shield and, therefore, the shield will introduce losses into this field which in turn prevent the flow ofinagnetic lines through the shield into the 001 In order to maintain the proper relation between the shield and the enclosed appai'atus and to impart rigidity to the structure, the core 1 is provided with a plurality of legs 9, preferably equally spaced on either end of the core, the legs being of a length sufficient to abut the inner surface of the shield.

' The coil may be mounted on a panel 10 by a, as strewn in 1,

It is to be further understood that the form of shield represented in the drawings illustrates only one term which this shield may take since in theory, the shield "for each element to be protected will have a separate and distinct shape out its own.

- lVhat is claimed is:

1. An electro-magnetic shield for electrical inductance apparatus having a surface including substantially all points at which equal potentials are set up by the flux of said;

apparatus.

2. A metallic shieldv tor inductance apparatus comprising a pluralityot parts so shaped as to include substantially allpoints at which, equal potcntialsiare setup by the V flux of said apparatus, said parts being; se-

cured together by, bands of insulating material.

A shield fODlIldLlCtZUlCGS. comprising a SUYIFHCG plotted so as to include in it sub- ,stantially all points at which exist equal potentials induced therein by the flux setup by said inductance.

a. In combination, an inductance, the elec- I trical energy in which sets up lines of torce,

a snield surrounding said inductance and lo \ment havingits surface formed so as to inelude substantially all points at which equal potentials are set up by the flux of said element,said shield being formed in a plurality of parts to prevent a continuous flow of current therein.

6. A magnetic shield "for an inductance element having its surface formed so as to include substantially all points at which equal potentials are set up by the flux of said ele ment, said shield being split transversely to prevent the flow of current caused by the inducing 01" a ditl'erence in potential at the ends of said shield.

7. A magnetic shield for an inifluctance element havingv its surface ttormcd so as to include substantially all points at which. equal potentials are setup by the tlux otsaid element, said shield being split longitudinally to prevent the flow of currents induced there- 8. A magnetic shield for an inductance element having its surface formed so as to include substantiallyall points at which equal potcntialsare setup by the flux of said element, said shield being split transversely to prevent the flow of current caused by the setting up ota ditierence in potential at the ends olisaid shield,-and said-shield beingsplit longitudinally to prevent the How of currents induced therein.

In testimony whereof, I have hereunto my hand.

I :MORRIS H. BENNETT. 

